Encyclopedia of the Solar System 2nd ed

Meteorites 265

(> 100 μm) feldspar characterize very rare type 7. These
pigeonholes approximate a chondritic thermal metamor-
phic continuum. Petrographic properties (with bulk carbon
and water contents) suggest increasing aqueous alteration
of type 3 material into types 2 and 1.
Two of these characteristics are illustrated in Fig. 12:
the opaque matrix and distinct chondrules of the type 3
chondrite Sharps (Fig. 12a) should be contrasted with the
recrystallized matrix and poorly defined chondrules of ex-
tensively metamorphosed (type 6) Kernouve (Fig. 12b).
Chemically, Fe^2 +contents of the ferromagnesian silicates—
olivine and pyroxene (Table 2)—are almost completely
random in a chondrite like Sharps and quite uniform in

one like Kernouve. Chondrites of higher numerical types

could acquire their petrographic characteristics (Table 4)

by extended thermal metamorphism of a more primitive

(i.e., lower type) chondrite of the same chemical group.

Temperature ranges estimated for formation of types 3–7

are 300–600, 600–700, 700–750, 750–950, and> 950 ◦C, re-

spectively.

The petrography of achondrites, like the martian mete-

orite Nakhla, clearly indicates igneous processes in parent

bodies at temperatures 1000 ◦C. The resultant melting

and differentiation erased all textural characteristics of the

presumed chondritic precursor (Fig. 12c) so its nature can

only be inferred.

TABLE 4 Definitions of Chondrite Petrographic Typesa

Petrographic Types

Uniform 1 2 3 4 5 6

(i) Homogeneity of

olivine and

pyroxene

compositions

— >5% mean deviations >5% mean

deviations to

uniform

(ii) Structural state of

low-Ca pyroxene

— Predominantly monoclinic Monoclinic Orthorhombic

>20% <20%

(iii) Degree of

development of

secondary feldspar

— Absent < 2 μm grains < 50 μm

grains

50- to 100-μm

grains

(iv) Igneous glass — Clear and isotropic primary

glass; variable abundance

Turbid if present Absent

(v) Metallic minerals

(maximum Ni

content)

—(<20%) Taenite

absent or

very minor

Kamacite and taenite present (>20%)

(vi) Sulfide minerals

(average Ni

content)

— >0.5% <0.5%

(vii) Overall texture No chondrules Very sharply defined chondrules Well-defined

chondrules

Chondrules

readily

delineated

Poorly defined

chondrules

(viii)Texture of matrix All

fine-grained,

opaque

Much opaque

matrix

Opaque matrix Transparent

microcrystalline

matrix

Recrystallized matrix

(ix)Bulk carbon

content

−3.5% 1.5–2.8% 0.1–1.1% <0.2%

(x) Bulk water

content

−6% 3–11% <2%

aThe strength of the vertical line is intended to reflect the sharpness of the type boundaries. A few ordinary chondrites of petrographic type 7 have

abnormally large (>100μm) feldspar and no chondrules: These have been interpreted as reflecting higher metamorphic temperatures than those associated

with type 6. Water contents do not include loosely bound, that is, terrestrial water (modified from Table 1.1.4 in “Meteorites and the Early Solar System).

Uniform